The heart is an intricately shaped, moving, three dimensional organ. Volumetric ultrasound imaging may generate images of the heart which may not be attainable with a two-dimensional ultrasound scanner. One of the challenges in generating volumetric ultrasound images is the high data acquisition rate which may be needed to scan the heart at a desired frame rate such as 30 frames per second (in real time).
In particular, ultrasound imaging may provide images of tissue in a body region by exciting an ultrasound transducer that generates ultrasound energy directed into the tissue. The ultrasound transducer, such as a piezoelectric crystal, may be excited with an electrical signal that produces a pressure wave which propagates into the tissue. As the propagating pressure wave encounters changes in the acoustic impedance of the tissue, a portion of the pressure wave is reflected back towards the ultrasound transducer which converts the reflected pressure wave back into an electrical signal for processing and display as part of an ultrasound image.
An ultrasound image of the tissue may be generated by electronically steering the pressure waves in the region by controlling the phasing of the excitations to a plurality of ultrasound transducer elements to form a transmit ultrasound beam. For example, electronic steering of transmit ultrasounds beams is discussed in U.S. Pat. No. 4,596,145 to Smith et al. In general, each excited ultrasound transducer element produces a corresponding pressure wave that is timed to constructively combine with pressure waves generated by other ultrasound transducer elements at a predetermined angle and range in the tissue. Consequently, phasing of the excitations to the plurality of ultrasound transducer elements may enable the transmit ultrasound beam to be steered within the region without moving the ultrasound transducer.
A number of the ultrasound transducer elements operate in a receive mode which receive the reflected pressure waves created by the corresponding transmit ultrasound beams and convert the pressure waves to electrical signals. The ultrasound system then adjusts the timing of the electrical signals that correspond to the reflected pressure wave to generate a receive ultrasound beam. Moreover, the ultrasound system processes (or focuses on) each reflected pressure wave dynamically (known as dynamic focusing). In particular, pressure waves reflected from points in the tissue located closer to the ultrasound transducer are reflected back and arrive at the ultrasound transducer earlier than points that are farther away from the ultrasound transducer.
Using dynamic focusing, the ultrasound system processes the earlier reflected pressure waves by focusing on the closer points first and then focusing on the farther points as time elapses. Thus the ultrasound system forms a receive ultrasound beam that corresponds to a transmit ultrasound beam by dynamically focusing on the reflected pressure waves created by the corresponding transmit ultrasound beam as the transmit ultrasound beam propagates in the tissue.
It is known to use parallel receive processing in conjunction with dynamic focusing to increase the data acquisition rate of the ultrasound imaging system. For example, parallel receive processing is discussed in U.S. Pat. No. 4,694,434 to von Ramm. In general, parallel receive processing may be performed in two stages. First, a broadened transmit beam is propagated into the tissue. The transmit ultrasound beam may be broadened by using a plurality of the ultrasound transducer elements adjacent to one another in the center of the ultrasound transducer as the transmitting elements. A broadened transmit ultrasound beam may insonify a larger amount of tissue in the body region. Second, parallel receive ultrasound beams are simultaneously acquired around the broadened transmit ultrasound beam.
Increasing the number of parallel receive ultrasound beams may, however, cause some of the receive ultrasound beams to under-steer the desired location. For example, broadening the transmit ultrasound beam may cause a decrease in the acoustic Signal-to-Noise Ratio (SNR) and a loss of resolution, thereby possibly degrading the image quality. The under-steer may be exacerbated as the receive ultrasound beams are placed farther from the center of the broadened transmit ultrasound beam. The under-steer may limit conventional volumetric ultrasound imaging systems to acquiring 16 parallel receive ultrasound beams around a single broadened transmit ultrasound beam.
It is known that improvements in the quality of ultrasound imaging may be achieved by increasing the frequency at which the ultrasound transducers described above are excited. For example, an ultrasound transducer array that is excited at 5.0 MHz may provide better image quality than an ultrasound transducer array that is excited at 2.5 MHz. Scanning a region using the higher frequency may, however, require more transmit ultrasound beams.
In particular, the frequency of the excitation may be proportional to the resolution of a scan so that, as the frequency increases, the resolution of the scan increases. Increasing the resolution of the scan may cause the spacing between adjacent transmit ultrasound beams decrease so that more transmit ultrasound beams may be needed to adequately scan the region. For example, conventional ultrasound systems using 2.5 MHz excitation may provide a lateral resolution of about 1.degree.. Increasing the frequency above 2.5 MHz may increase the resolution to less than 1.degree.. Consequently, the data acquisition rate of an ultrasound imaging system that generates transmitted ultrasound beams above 2.5 MHz may need to be increased to maintain adequate scanning.
The data acquisition rate of conventional ultrasound imaging systems may also define the size of the region that can be imaged in real time. In particular, increasing the data acquisition rate may allow the size of the imaged region to be increased while maintaining a desired frame rate. For example, increasing the data acquisition rate may allow an increase in a volume scan angle from 60.degree. to 80.degree. to enable the display of the four heart chambers in the apical view. Alternatively, increasing the data acquisition rate may be used to provide deeper scans while maintaining a desired frame rate. Accordingly, there continues to exist a need to further increase the data acquisition rate of ultrasound imaging systems.